1. Field of the Invention
The invention relates generally to industrial security and more specifically to devices for the detection of intruders in interior areas.
2. Description of the Prior Art
In the prior art there have been various devices developed for detecting the entry of an intruder into a building, such devices frequently involve window and door security devices, or the like, which simply detect the entry of the intruder, but take no note of his presence once he is inside the secured area. Thus, if the intruder is able to deceive the entry detectors, he is relatively free to move about inside the supposedly secure area without detection.
Another class of conventional commercial radio frequency intrusion detection devices known in the prior art makes use of the Doppler principle to detect a moving intruder. In such systems an unmodulated radio frequency signal is transmitted (typically at a power level on the order of 30 MW and in the general frequency area of 10 GHz). The reflection from an intruder is mixed with a sample of the transmitted signal and the resultant Doppler is fed to a processor. The processor may simply be a filter and detector or a balance processor which attenuates moving clutter. Maximum Doppler shift at 10 GHz will range from 2 Hz to 400 Hz for target motions of 0.1 ft. per second to 20 ft. per second, respectively, and the minimum Doppler shift will be 0.
The required variations in coverage are generally obtained by changing antenna beam shape (to limit angle coverage) and by reducing transmitter power or receiver gain to limit the detection range.
Although the above described existing systems are relatively simple and inexpensive, they have important limitations with respect to large area motion detection.
The coverage requirement is generally for a rectangular area, and, since the area is illuminated by an antenna beam solid angle, the entire rectangular area is not uniformly covered. If the antenna angle is broadened to increase the coverage, then the beam is longer contained within the rectangle and detection outside the area of interest results. Furthermore, there is no sharp detection cutoff at the edges of the beam as a function of distance, for example, if the system is set up such that a human target is reliably detected at maximum range, a target which is 12db stronger (e.g., a large moving truck) will be seen at twice the maximum range. Targets that are larger than the human intruder, but not as large as the moving truck, will be detected at ranges between the desired maximum range and twice the desired maximum range. Thus, there is no positive containment at the rectangle edges and the system is susceptible to false alarms. Most of the commerical systems presently extant operate at higher microwave frequencies (typically 10 GHz), and require several antenna installations to cover the area of interest. This increases the cost and complexity. The higher microwave frequencies generally used are selected in preference to lower frequencies so that antenna arrays are of very moderate size. The choice of a 10 GHz operating frequency, rather than a lower frequency on the order of 1 GHz, has certain disadvantages however; among them being the fact that a human target radio echo is several decibels lower at the higher frequency than at the lower frequency. Moreover, at the higher frequency, the problem of shadowing in signal attenuation within the building due to obstructions such as cargo, shelves, walls, etc., is much more severe. Still further, typical target Doppler frequencies fall between 2 Hz and 400 Hz when the 10 GHz frequency is used, and therefore, the signals to which the system must repond occupy a requency band which also contains the ordinarily used power sources of 60 Hz and 400 Hz.
The general techniques of pseudo-random coded sequence (sometimes referred to as a pseudo-noise sequence) radar employed in the combination of the invention are variously described in the technical literature. For exmaple, the text "Radar Handbook", by Merrill Skolnik (McGraw Hill, 1970) describes with bibliographic references, the biphase coded sequences required. See Chapter 20, Sec. 20.5 of that text. Additional description for an understanding of pseudo-noise techniques is also found in two other texts, namely; "Radar Design Principles", by F. Nathanson, (Chapter 12, Page 452) and "Modern Radar", by R. S. Borkowitz, (Chapter 4, Page 247).